Apparatuses and methods for efficient write in a cross-point array

ABSTRACT

A memory circuit, including a memory array (such as a cross-point array), may include circuit elements that may function both as selection elements/drivers and de-selection elements/drivers. A selection/de-selection driver may be used to provide both a selection function as well as an operation function. The operation function may include providing sufficient currents and voltages for WRITE and/or READ operations in the memory array. When the de-selection path is used for providing the operation function, highly efficient cross-point implementations can be achieved. The operation function may be accomplished by circuit manipulation of a de-selection supply and/or de-selection elements.

CROSS REFERENCES

The present Application for Patent is a continuation of U.S. patentapplication Ser. No. 15/237,363 by Castro, entitled “Apparatuses andMethods for Efficient Write in a Cross-Point Array,” filed Aug. 15,2016, which is a continuation of U.S. patent application Ser. No.14/591,800 by Castro, entitled “Apparatuses and Methods for EfficientWrite in a Cross-Point Array,” filed Jan. 7, 2015, which is acontinuation of U.S. patent application Ser. No. 13/914,170 by Castro,entitled “Apparatuses and Methods for Efficient Write in a Cross-PointArray,” filed Jun. 10, 2013, assigned to the assignee hereof, and eachof which is expressly incorporated by reference in its entirety herein.

BACKGROUND Field of the Invention

Embodiments of the present disclosure generally relate to electronics,and in particular, to memory circuits.

Description of the Related Art

Memory for computers or other electronic devices can include blocks ofmemory cells integrated into a larger integrated circuit or standaloneintegrated circuits. There are many different types of memory includingrandom-access memory (RAM), read only memory (ROM), dynamic randomaccess memory (DRAM), static RAM (SRAM), synchronous dynamic RAM(SDRAM), flash memory, magnetoresistive random-access memory (MRAM) andphase change memory (PCM). Many semiconductor memory technologiesarrange individual memory cells into arrays that may be accessed usingbit lines (or column select lines) that connect to columns of memorycells in the array and word lines (or row select lines) that connect torows of memory cells in the array.

Many types of memory may be accessed by providing a select voltage onthe appropriate bit line and/or word line. Some memory technologies,such as resistive cross-point memory technologies including PCM, may usea deselect voltage to be driven on the inactive word lines and/or bitlines to achieve proper biasing during a memory access.

In certain instances, a memory cell may comprise a PCM component (forexample, a germanium-antimony-tellurium (GST) component such as anovonic memory switch (OMS), etc.) and a selection component (forexample, a thresholding component such as an ovonic threshold switch(OTS)). Such a memory cell may, for example, be referred to as a PCM andSwitch (PCMS) memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings and the associated description herein are provided toillustrate specific embodiments of the disclosure and are not intendedto be limiting.

FIG. 1A is a circuit diagram showing a memory array with selection andde-selection elements in accordance with one or more embodiments of thedisclosure.

FIG. 1B is a circuit diagram showing a memory cell with array driverswhich may include selection and de-selection elements in accordance withone or more embodiments of the disclosure.

FIG. 1C is a chart showing circuit behavior during a SET function when aset current is provided by a selection supply, in accordance with one ormore embodiments of the disclosure.

FIG. 1D is a chart showing circuit behavior during a SET function when aset current is provided by a de-selection supply, in accordance with oneor more embodiments of the disclosure.

FIG. 2A illustrates an example of an array driver with a fully decodedde-selection element, in accordance with one or more embodiments of thedisclosure.

FIG. 2B illustrates an example of an array driver with partially decodedde-selection elements, in accordance with one or more embodiments of thedisclosure.

FIG. 3A illustrates an example of a non-transistor de-selection element,in accordance with one or more embodiments of the disclosure.

FIG. 3B illustrates an example implementation of a non-transistorde-selection element, in accordance with one or more embodiments of thedisclosure.

FIG. 3C illustrates an example of a non-decoded de-selection element, inaccordance with one or more embodiments of the disclosure.

FIG. 4 illustrates an example of an array-based de-selection element, inaccordance with one or more embodiments of the disclosure.

FIGS. 5A-5B are a circuit diagrams showing a memory arrays withde-selection elements including multiple de-selection supplies, inaccordance with one or more embodiments of the disclosure.

FIG. 6 is another chart showing circuit behavior during a SET function,in accordance with one or more embodiments of the disclosure.

FIG. 7 is yet another chart showing circuit behavior during a SETfunction, in accordance with one or more embodiments of the disclosure.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Although particular embodiments are described herein, other embodimentsof the invention, including embodiments that do not provide all of thebenefits and features set forth herein, will be apparent to those ofordinary skill in the art.

A memory circuit, including a memory array (such as a cross-pointarray), may include circuit elements that may function both as selectionelements/drivers and de-selection elements/drivers. Aselection/de-selection driver may be used to provide both a de-selectionfunction, as well as a selection function. The selection function mayinclude providing sufficient currents and voltages for WRITE and/or READoperations in the memory array. The selection function may beaccomplished by circuit manipulation of a de-selection supply and/orde-selection elements, as is described in detail below.

In one embodiment, efficient biasing in a memory array (such as across-point array) of cells (for example, PCM cells) can use de-selectedcells configured with a relatively low amount of electricalpotential/voltage across them. This is accomplished by driving the wordlines and bit lines associated with the de-selected cells in the arrayto a “mid-bias” voltage level with respect to the selectedpotentials/voltages of the memory array/circuit.

One approach to achieving the de-selection of cells, or the de-selectionfunction, is to provide a de-selection driver at each, for example, wordline and bit line of the memory array. The de-selection driver maycomprise, for example, a transistor, a reduced function transistor, apassive element, and/or an array based element. Alternative approachesalso exist whereby the selection function, as well as the de-selectionfunction, is accomplished by the same transistor.

Various embodiments of the present disclosure describe methods andcircuit apparatuses including selection/de-selection elements and/ordrivers that may provide both the de-selection function and theselection function.

In an embodiment, transistor-based selectors/de-selectors may includefull decoding. In this embodiment, a connection to theselector/de-selector is accomplished automatically by applying aselection pulse to the target memory cell, followed quickly by ade-selection transition on one electrode (word line or bit line). Whenthe selection pulse is sufficient to snapback the memory cell (i.e.,cause a snapback event), the memory cell remains in the on state (alsoreferred to as the active state or selected state) through thede-selection transition. Alternatively, the memory cell may briefly turnoff during the de-selection transition, but may be re-triggered during arecovery period of the cell. For some memory technologies, includingphase change materials, the threshold of the memory cell immediatelyafter trigger is reduced and may require a finite period of time beforerecovering to its initial value. The de-selection transition maycomprise supplying voltage and current to the selected cell from amid-bias (or de-selection) source/supply. Following the de-selectiontransition, the memory cell may be selected and/or a SET operation maybe accomplished (and/or other WRITE and/or READ operations may beaccomplished) as a particular (and sufficient) current is supplied tothe cell for a particular (and sufficient) period of time. In thisembodiment, no additional selection pulse circuitry is needed other thanthe circuitry necessary to generate the actual selection pulse. Inaddition, in various embodiments, the mid-bias supply may not bedecoded. High efficiency is achieved compared to methods that use theselection path for the SET operations because the mid-bias supply is alower voltage supply than the selection pulse supply.

In various embodiments, odd/even or other reduced decoding of thede-selection path may be accomplished with any of waffle, passive,and/or array-based devices. In these embodiments there may be a slightdelay in enabling the de-selection path due to, for example, circuitmismatches. The de-selection transition may be accomplished as long asthe de-selection path is enabled within the recovery period of thememory cell, and the de-selection (mid-bias) supply is capable ofsupporting the currents necessary for the WRITE and/or READ operation.

In various embodiments, the de-selection (mid-bias) supply may belimited in supply capacity, and may not provide sufficient voltageand/or current to the memory cell for WRITE and/or READ operations. Inthese embodiments, the mid-bias supply may be partially decoded toenable the memory array to meet WRITE and/or READ operationrequirements. A mid-bias supply being partially decoded may, in variousembodiments, mean that that one or more mid-bias supplies may beselectively applied to one or more groups of bit lines and/or wordlines. For example, in an embodiment, partial decoding of the mid-biassupply may comprise implementing multiple mid-bias supplies so as toincrease the current available from each. Further, groups of bit and/orword lines may each be electrically coupled to one of the multiplemid-bias supplies. In an embodiment, the voltage of one or more of themid-bias (de-selection) supplies may be increased temporarily so as tomeet current requirements for WRITE and/or READ operations.

In various embodiments, array-based de-selector elements may be used. Inthese embodiments, the array-based de-selector elements may havethreshold characteristics. Following the de-selection transition, apartially decoded de-selection supply may provide a voltage pulse to oneor more of the array-based de-selector elements to trigger the elementsinto a low-resistance state (due to a snapback event). The lowresistance state of the one or more array-based de-selector elements maythen provide additional current necessary to support WRITE and/or READoperations in the selected memory cells.

Various embodiments of the present disclosure advantageously usesignificantly less energy and power because current used during WRITEand/or READ operations is supplied by the mid-bias supply, rather thanthe much higher voltage selection supply. Additionally, because currentis supplied to the memory cell from the selection supply onlytemporarily, particular elements of the selection circuitry (forexample, particular transistors) no longer must supply high current, andmay thus be sized smaller than previously. The smaller transistor sizemay, for example, reduce on-chip area requirements for the memorydevice. Additionally, because the voltages used during the WRITE and/orREAD functions are lower, the transistor critical dimensions such asgate-length and gate oxide thickness may be reduced, further reducingthe on-chip area required. Lower voltages may further enableimprovements in power, speed, and/or resiliency of the memory device.Potentially damaging electrical fields may also be reduced as a resultof the use of lower voltages.

FIG. 1A is a circuit diagram of a memory circuit 1000 with a memoryarray 1002 and associated select/de-select drivers (for example,select/de-select drivers 1044, 1046). The memory array 1002 may comprisean array of memory cells 1004. Any type of memory technology may be usedincluding, but not limited to, dynamic random access memory (DRAM),flash memory, and phase change memory (PCM), memristor memory, or othertypes of memory. Although FIG. 1 shows a 4×4 array of 16 memory cells1004, the memory array 1002 may contain any number of memory cells 1004,and may not have the same number of columns as rows. The memory array1002 may typically contain at least many millions of memory cells 1004;however, disclosed subject matter is not limited to a particular number.The memory array 1002 may have a number of select lines that may crossthe memory array 1002 in two directions. The select lines may bereferred to as row select lines, or word lines 1022, and column selectlines, or bit lines 1020, although other names may be used. Respectivememory cells 1004 in the memory array 1002 may be coupled to one wordline 1022 and one bit line 1020. For example, by applying a selectvoltage on a word line and a select voltage on a bit line, the memorycell 1004 that is coupled to those two lines may be accessed to read orwrite the memory cell 1004. The exact voltage and timing sequence forthe select lines may vary depending on the memory technology used, andsome memory technologies may use additional lines to access anindividual memory cell 1004 or group of memory cells 1004.

Select/de-select drivers in the memory circuit 1000 may be operable toboth select and de-select individual memory cells 1004, as is describedin detail below. For clarity purposes, the present disclosure may usethe following terms interchangeably, all of which are intended to referto embodiments of select/de-select drivers: “driver,” “select/de-selectdriver,” “select driver,” “de-select driver,” “selection/de-selectiondriver,” “selection driver,” and/or “de-selection driver.” Additionally,the terms “driver” and “element” may be used synonymously andinterchangeably. Thus, the term “de-selection element,” among others,may also be used to refer to embodiments of the select/de-select driversof the present disclosure.

An address 1018 may be provided by a memory controller (not shown) andreceived by the memory circuit 1000 and distributed to column circuitry1014 and row circuitry 1016 to be decoded. Sections of the address 1018may be decoded in various ways in one or more embodiments. Each of therow circuitry 1016 and column circuitry 1014, as described herein, maybe implemented using similar embodiments or using different techniques.

In some embodiments, the row circuitry 1016 and column circuitry 1014may fully, or nearly fully, decode the address to provide 2 ^(n)select/de-select control lines 1040, 1042 for n address bits that may beused to control the select/de-select drivers (for example,select/de-select drivers 1044, 1046) so that for each row and column, atleast one select/de-select driver is enabled to select a line and the atleast one or more other select/de-select drivers enabled to de-selectlines. In other embodiments, a section of address may be split intomultiple subsections to provide a cascaded decode. As an example, an8-bit section of address 1018 for use in selecting a particular wordline 1022 or bit line 1020 of the 256 word lines or 256 bit lines of thememory array 1002 may be split into two 4-bit subsections. The low order4 bits may be decoded into 16 select/de-select control lines 1040 orcontrol lines 1042 that drive 16 sets of 16 select/de-select drivers(for example, select/de-select drivers 1044, 1046) so that one or moreof the 16 outputs drive 16 select/de-select drivers. The row circuitry1016 and column circuitry 1014 of FIG. 1A each shows two bits of addressdecoded into four control lines 1040, 1042, however, in variousembodiments larger or smaller decoder circuitry may be implemented forany number of address bits.

In an embodiment, a word line select/de-select driver 1044 may include ap-type field-effect transistor (FET) 1024 and an n-type FET 1026. Thefield-effect transistors (FETs) or “transistors” described herein cancorrespond to transistors known as metal-oxide-semiconductorfield-effect transistors (MOSFETs). While the terms “metal” and “oxide”are present in the name of the device, it will be understood that thesetransistors can have gates made out of materials other than metals, suchas polycrystalline silicon, and can have dielectric “oxide” regions madefrom dielectrics other than silicon oxide, such as from silicon nitrideor high-k dielectrics. The gates of the p-type FET 1024 and n-type FET1026 may be driven by the appropriate control line 1040 output by therow circuitry 1016. The drain of the p-type FET 1024 and the drain ofthe n-type FET 1026 may be coupled to the appropriate word line 1022.Additionally, the source of the p-type FET 1024 may be coupled to ade-select word line voltage source (DESWL) 1012, also referred to as amid-bias voltage source, while the source of the n-type FET 1026 may becoupled to a ground 1008 (such as, zero volts) or other voltagereference. Similarly, other word lines 1022 may be coupled to respectiveselect/de-select drivers as shown in FIG. 1A.

Additionally, a bit line select/de-select driver 1046 may include ap-type FET 1034 and an n-type FET 1032. The gates of the p-type FET 1034and n-type FET 1032 may be driven by the appropriate control line 1042output by the column circuitry 1014. The drain of the p-type FET 1034and the drain of the n-type FET 1032 may be coupled to the appropriatebit line 1020. Additionally, the source of the p-type FET 1034 may becoupled to a select bit line (SELBL) voltage source 1006, while thesource of the n-type FET 1032 may be coupled to a de-select bit line(DESBL) voltage source 1010, also referred to as a mid-bias voltagesource. Similarly, other bit lines 1020 may be coupled to respectiveselect/de-select drivers as shown in FIG. 1A.

The SELBL voltage source 1006 may be referred to as a selection source,a selection supply, a selection pulse source/supply, and/or a highvoltage source/supply. Further, the voltage supplied by the SELBLvoltage source 1006 may be referred to as a selection voltage and/or aselection potential. In an embodiment, the SELBL voltage source 1006 maysupply a voltage of 8 volts. In an embodiment, the mid-bias voltagesources, for example, the DESBL voltage source 1010 and/or the DESWLvoltage source 1012, may supply a voltage of 4 volts. Alternativevoltage ranges are possible such as (6 v, 3 v, 0 v) or (−5 v, 0 v, 5 v)respectively, as necessary to meet the biasing requirements of thecross-point array of the particular technology. Other voltages may besupplied in the memory circuit 1000, depending on the memory technologyof the particular embodiment. In an embodiment, the voltages andcurrents supplied to the memory circuit 1000 maybe provided by off-chipsources through, for example, pins. Alternatively, the voltages andcurrents may be provided by on-chip sources.

The select/de-select drivers of the memory circuit 1000 may beconstructed using any applicable technology including metal oxidesemiconductor field-effect transistors (MOSFET), chalcogenide devices,bipolar junction transistors, or other types of electronic devices. Theselect/de-select drivers may be designed to handle the voltage andcurrent requirements for selecting a memory cell 1004 or the memoryarray 1002 which may depend at least in part on the memory technologyused.

The operation of the memory circuit 1000 is described below in referenceto FIGS. 1B-1D and the remaining figures.

FIG. 1B shows a simplified circuit diagram of an illustrative memorycell circuit 102, which includes components that can be shared withother memory cells, such as a driver circuit 112, 118, in accordancewith one or more embodiments of the disclosure. For example, the firstdriver 112 can be shared by a plurality of memory cells of a column ofan array, and the second driver 118 can be shared by a plurality ofmemory cells of a row of the array. In an alternative embodiment, thefirst driver 112 can be used to drive a row, and the second driver 118can be used to drive a column. The memory cell circuit 102 of FIG. 1Bincludes a memory cell 122, which may correspond to any of the memorycells 1004 of FIG. 1A.

The memory cell circuit 102 includes a node 104 for a first voltagereference, a node 106 for a second voltage reference, and a node 108 fora third voltage reference. In the illustrated embodiment, the firstvoltage reference corresponds to a select bit line (SELBL) voltagesource 104, the second voltage reference corresponds to a mid-biasvoltage source 106, and the third voltage reference corresponds toground 108. In the illustrated embodiment, the SELBL voltage is positiveand higher than the mid-bias voltage, which is positive and non-zero,and ground is at 0 volts. However, other voltages can be used for thevoltage references.

The mid-bias voltage source 106 may correspond to either or both of theDESBL voltage source 1010 and DESWL voltage source 1012 of FIG. 1A.Additionally, the memory cell circuit 102 includes a bit line (BL)select 110 corresponding to, for example, one of the control lines 1042(FIG. 1A), a word line (WL) select 116 corresponding to, for example,one of the control lines 1040 (FIG. 1A), a first driver 112corresponding to, for example, the bit line select/de-select driver 1046(FIG. 1A), a second driver 118 corresponding to, for example, the wordline select/de-select driver 1044 (FIG. 1A), a bit line 114, a word line120, and the memory cell 122.

In an embodiment, for example in PCMS technology, the memory cell 122may comprise a memory element and a selection component. Such a memorycell 122 may exhibit snapback behavior or have snapback events undercertain conditions, and may be referred to as a snapback device. Asnapback event may result in a sudden “negative resistance”, undercertain conditions. An occurrence of a snapback event tends tosignificantly affect a current-voltage behavior of a memory cell. Forexample, a snapback event generally drives the device into a highconduction and/or low resistance state. Such a snapback event may besensed or detected. Thus, in various embodiments, a sense circuit may,for example, be provided which is responsive to a snapback eventoccurrence in a memory cell to generate one or more feedback signals. Byway of example, one or more feedback signals may initiate a change in anelectric potential across the memory cell and/or memory element toreduce the electric potential, disconnect the electric potential, stopthe generation of the electric potential, etc. Thus, for example, thepower consumption of a memory cell may be reduced in response to asnapback event.

In the memory cell circuit 102 of FIG. 1B, the memory cell 122 may beselected and/or de-selected by the first driver 112 and the seconddriver 118, as is described below. Selection of the memory cell 122 maybe accomplished to, for example, establish a programmed state in thememory cell (for example, as part of a WRITE operation). Examples ofwrite operations may include, for example, SET and RESET. Selection ofthe memory cell 122 may also be accomplished to, for example, determinea programmed state of the memory cell (for example, as part of a READoperation). On the other hand, de-selection of the memory cell 122 may,depending on the memory technology, be accomplished so as to allowselection of other memory cells for WRITE and/or READ operations. Thememory cell circuit 102, including the first driver 112 and the seconddriver 118, enable both selection and de-selection of the memory cell122 without any additional circuitry. Additionally, as is describedbelow, the arrangement of the memory cell circuit 102 may significantlyreduce the power required to perform WRITE and/or READ operations.

FIG. 1C illustrates a set of voltage waveforms showing circuit behaviorfor a memory cell during a SET function when a set current is providedby a selection supply. FIG. 1C may be contrasted with the voltagewaveform of FIG. 1D, which is described in detail below. In particular,the voltage waveform of FIG. 1C shows a typical SET function of thememory cell circuit 102 that may use a significant amount of energy. Onthe other hand, FIG. 1D shows a SET function of the memory cell circuit102 (FIG. 1B) that may use significantly less energy by making use ofthe mid-bias source 106, according to an embodiment of the presentdisclosure.

FIG. 1C shows four voltage waveforms 150, 152, 154, 156 with voltagesvarying over time between zero volts and the SELBL voltage, withmid-bias indicating some voltage between the SELBL voltage and zerovolts. Typically, the mid-bias voltage is a voltage substantiallyhalf-way between SELBL and zero volts. Also shown in FIG. 1C is thestate of WL selection 168 (corresponding to WL select 116) and BLselection 170 (corresponding to BL select 110).

In FIG. 1C, a waveform 150 for the voltage of the bit line 114 and awaveform 152 for the voltage of the word line 120 for the selectedmemory cell are indicated. A waveform 156 for the voltage of a bit lineand a waveform 154 for the voltage of a word line for one or morede-selected memory cells are also indicated in FIG. 1C. As shown, thevoltages of both the bit and word lines of the de-selected cells(waveforms 154 and 156) are driven to the mid-bias level through theentire time period represented. On the other hand, beginning at a timeinstant represented by a dashed line 169, when the WL selection 168 goeshigh and the BL selection 170 goes high, the waveform 150 indicates thatthe bit line 114 is pulled to the SELBL voltage, while the waveform 152indicates that the word line 120 is pulled to zero volts. As shown inthe time period 158, a snapback event rapidly occurs in the selectedmemory cell, causing a collapsed voltage.

During a time period 160, the selected memory cell enters a stable stateduring which the cell may be set. In the example of FIG. 1C, adifference 162 in the voltage of the bit line 114 is indicatedcorresponding to a difference between when the memory cell is initiallyselected and after the snapback event occurs. Similarly, a difference164 in the voltage of the word line 120 is indicated corresponding to adifference between when the memory cell is initially selected and afterthe snapback event occurs. The voltage differences 162 and 164 representresistive voltage drops across the array driver transistors andelectrodes in the array. During the stable state time period 160, thevoltage difference across the memory cell 122 (between the bit line 114and word line 120) is indicated by a difference 166. The difference 166also provides an indication of the current flowing from the SELBLvoltage source 104 to ground, and the amount of power being used by thememory cell during the SET operation. In an embodiment, the memory cellis set by the application of a particular magnitude of current over aparticular period of time. Thus, the memory cell may be set by thecurrent applied to the memory cell 122 during the time period 160. Invarious other embodiments, the shape of the waveforms during the SETfunction may be any other shape besides the generally rectangular shapeof the waveforms of FIG. 1C during time period 160. For example, theshape of the waveforms may include one or more pulses, and/or may beramped.

At the end of the SET function, the bit line 114 and word line 120 ofselected memory cell settle back to the mid-bias supply voltage.

Turning now to FIG. 1D, as mentioned above, FIG. 1D shows a SET functionof the memory cell circuit 102 that may use significantly less energyrelative to the function of FIG. 1C by making use of the mid-bias source106, according to an embodiment of the present disclosure. FIG. 1D againincludes waveforms 150, 152, WL selection 168, and other (unlabeled)waveforms representing other de-selected memory cells. FIG. 1D alsoincludes a BL pulse 182. At a time period represented by a dashed line180, the memory cell 122 is selected as indicated by the activation ofthe WL selection 168 and the BL pulse 182, at which point the bit line114 is pulled to the SELBL voltage, and the word line 120 is pulled toabout zero volts. During a time period 171, a snapback event quicklyoccurs, at which point the voltage across the memory cell 122 collapses.

At a time instant represented by a dashed line 172, immediately afterthe snapback event occurs, the BL select 110 is de-selected, asindicated by the BL pulse 182. The correct timing of the de-selection ofthe BL select 110 may be determined based on a period of time (forexample, a particular period of time after memory cell selection) and/orby detection of the snapback event. De-selection of the BL select 110biases the bit line 114 to the mid-bias level, rather than to the SELBLvoltage, as was the case described earlier in connection with FIG. 1C.For example, de-selection of the BL select 110 creates a de-selectionvoltage potential, or mid-bias voltage potential, that is, from mid-biasto zero volts), across the memory cell 122. Thus, during the stable timeperiod 174, the memory cell 122 biased between the mid-bias supply andground, rather than between the SELBL voltage and ground as describedearlier in connection with FIG. 1C, thereby saving power. Additionally,the snapback effect remains in effect, causing a further voltage drop ofthe bit line 114 and word line 120, indicated by a difference 175 (thedifference in voltage between the mid-bias and the bit line 114), adifference 178 (the voltage difference across the memory cell 122), anda difference 176 (the difference in voltage between the word line 120and ground). It should be noted that the differences 175 and 176 aresubstantially reduced from the equivalent differences 162 and 164, whilethe difference 178 (the voltage across the cell) is the same as thedifference 166.

As described in reference to FIG. 1C, during the time period 174 thememory cell is set by the current flowing through the memory cell 122.At the end of the SET function, the bit line 114 and word line 120 ofselected memory cell settle back to the mid-bias supply voltage.

In an embodiment, the time period 160 and/or time period 174 comprises aduration of on the order of hundreds of nanoseconds. In an embodiment,the current flowing through the memory element during the WRITEoperation may be on the order of tens of micro amps.

In various embodiments, the memory device is designed such that thevoltage difference 178 is sufficient to support the current that shouldflow through the memory cell to set the cell, taking into account anyadditional resistive paths (and/or leakage current). Alternatively,additional mid-bias supplies (for example, multiple DESBL voltagesources 1010 and/or DESWL sources 1012) may be provided to supportcurrent requirements (as described in reference to FIGS. 5A and 5Bbelow).

In various embodiments, the operation described in reference to FIG. 1Dmay be adapted to other WRITE and READ operations of the memory device.For example, the operation of FIG. 1D may be used in a RESET operation.In addition, the same operations can be extended to memory technologiesthat may require a polarity reversal of electrodes across the cell forthe some cell operations.

The operation of the memory cell as shown in FIG. 1D enables multipleadvantages over the operation shown in FIG. 1C. In particular, theoperation of FIG. 1D uses significantly less energy and power as thecurrent used to set the memory cell is supplied by the mid-bias supply,rather than the much higher SELBL supply. Additionally, because thecurrent is supplied to the memory cell from the SELBL supply onlytemporarily, the p-type FET 1034 (FIG. 1A) can be sized smaller thanpreviously. The smaller transistor size may, for example, reduce on-chiparea requirements for the memory device saving manufacturing cost.Additionally, because the voltages used during the WRITE and/or READfunctions are lower, transistor features such as gate-length andgate-oxide thickness may be reduced, further reducing the on-chip arearequired. Lower voltages may further enable improvements in power,speed, and/or resiliency of the memory device. Potentially damagingelectrical fields may also be reduced as a result of the use of lowervoltages.

The techniques described herein may variously be applied to all types ofcross point memories, including resistive memory arrays. In contrast totypical selection and de-selection techniques, in various embodimentsthe techniques of the present disclosure utilize combinedselection/de-selection drivers/elements to both select particular cells,and de-select other cells. These techniques enable many of theadvantages mentioned above, and further described below.

FIG. 2A illustrates an example of an array driver with a fully decodedselection/de-selection driver/element 202, in accordance with one ormore embodiments of the disclosure. The driver of FIG. 2A is designedfor driving word lines (similar to the word line select/de-select driver1044 of FIG. 1A), however a similar driver may be implemented fordriving bit lines.

In FIG. 2A, the driver 202 is fully decoded, meaning that each word linemay be separately selected/de-selected. FIG. 2A illustrates a DESWLsource 204 and a zero volt reference (or ground) 206 that are not partof the driver 202. The driver 202 includes a first local word line(LWL0) 208, a second local word line (LWL1) 210, a first word controlline (WL0) 209, a second word control line (WL1) 211, p-type FETs 212,216, and n-type FETs 214, 218, 224, 226. The DESWL source 204 (asgenerally described in reference to DESWL 1012 of FIG. 1A) provides amid-bias voltage source to the driver 202. Control signals for the WL0209 and WL1 211 can be provided by a memory controller (not shown). TheLWL0 208 and the LWL1 210 are generally coupled to many memory cells.The WL0 209 and the WL1 211 are coupled to control circuitry whichprovides control signals to the driver 202. The N-type FET 224 and then-type FET 226 may be used in some embodiments for additional decodingand/or control.

In operation, when at least one of the WL0 209 and/or the WL1 211 isde-selected or “low” in this example, the LWL0 208 and/or the LWL1 210is pulled to the level of the DESWL source 204 (mid-bias), respectively.This is indicated by the arrow 220 which indicates the de-selectioncurrent path. Alternatively, when the WL0 209 and/or the WL1 211 isselected or “high” in this example, the LWL0 208 and/or the LWL1 210 ispulled to the level of ground, respectively. This is indicated by thearrow 222 which indicates the selection current path.

FIG. 2B illustrates an example of an array driver with partiallydecoding (or partially decoded) de-selection driver 230, in accordancewith one or more embodiments of the disclosure. The illustratedembodiment of the driver of FIG. 2B is intended to drive word lines;

however, a similar driver may be implemented for driving bit lines. Manyof the same elements are from FIG. 2A are included in FIG. 2B, and thusonly differences between the two figures are described.

The driver 230 is partially decoding, meaning that groups of word linesmay be simultaneously de-selected (connected to the DESWL supply), whilespecific word lines may be selected as before. For example, in onearrangement, a single word control line may de-select many even and/orodd local word lines. As shown, driver 230, a single word control line(WL) 240 coupled to p-type FET 234 may also be coupled to one or moreother p-type FETs. Similarly, a single word control line coupled top-type FET 232 may also be coupled to one or more other p-type FETs.Thus, a signal provided by a memory controller and carried by the singleword control line (WL) 240 may de-select LWL1 210 and one or more otherlocal word lines, while separate signals coupled to n type FET 236 and238 may individually select the same local word lines.

FIG. 3A illustrates an example of a non-transistor (such as, passiveelement) selection/de-selection driver/element 300, in accordance withone or more embodiments of the disclosure. The illustrated embodiment ofthe driver of FIG. 3A is intended to drive word lines; however, asimilar driver may be implemented for driving bit lines. Many of thesame elements from FIG. 2B are included in FIG. 3A, and thus onlydifferences between the two figures are described.

In contrast with the drivers 202, 230, the driver 300 includes passiveelements. Thus, the driver 300 includes passive elements 302, whichcouple the DESWL source 204 to the LWL0 208 and the LWL1 210. Passiveelements 302 may include, in various embodiments, resistors and/ordiodes, among other elements. In operation, when one of the first wordcontrol line (WL0) 304 or the second word control line (WL1) 306 isde-selected, the LWL0 208 or the LWL1 210, respectively, is pulled tothe DESWL voltage. Control signals applied to the WL0 304 and the WL1306 can be provided by a memory controller. When either of the WL0 304and/or the WL1 306 is selected, the LWL0 208 and/or the LWL1 210 arepulled to ground, respectively.

FIG. 3B illustrates an example implementation of a non-transistorde-selection element layout 330, as may be used in the driver 300 ofFIG. 3A, in accordance with one or more embodiments of the disclosure.In the layout 330, each of contacts 334 may be coupled to a word and/orbit line (for example, LWL0 208 and/or LWL1 210), while each of thecontacts 332 may be coupled to a voltage source (for example, DESWL204). The area between each of contacts 332, 334 may comprise aresistive material. Thus, layout 330 comprises as distributed resistancebetween the voltage source and each local word line. In variousembodiments, other arrangements may be used to implement the driver 300.

FIG. 3C illustrates an example of a non-decoded, transistor-basedde-selection driver/element 340, in accordance with one or moreembodiments of the disclosure. The driver of FIG. 3C is designed fordriving word lines, however a similar driver may be implemented fordriving bit lines. Many of the same elements from FIG. 3A are includedin FIG. 3C, and thus only differences between the two figures aredescribed.

In contrast to drivers 202, 230, driver 340's de-selection path isnon-decoded. Thus, driver 340 does not enable selective de-selection ofvarious local word lines. The driver 340 includes a current mirrorcomprising p-type FETs 341, 344, 346, and line 342. The current minor iscoupled to each of the local word lines to provide current when thoselocal word lines are de-selected or selected.

FIG. 4 illustrates an example of an array-based de-selectiondriver/element 400, in accordance with one or more embodiments of thedisclosure. The driver of FIG. 4 is designed for driving word lines,however a similar driver may be implemented for driving bit lines. Manyof the same elements from FIG. 3A are included in FIG. 4, and thus onlydifferences between the two figures are described.

Similar to driver 340 of FIG. 3C, driver 400's de-selection path isnon-decoded. Driver 400 includes DESWL voltage sources 204 coupled tolocal word lines through array elements 402. In an embodiment, arrayelements 402 may comprise modified memory elements. Modified memoryelements may include modifications that are specific to a technology,and may include, for example, using larger dimensions than normal memoryelements, multiple memory elements connected in parallel, and/or memoryelements with modified switch components. In addition, the modifiedmemory elements may retain the snapback characteristic of the mainmemory elements.

In various embodiments, although the drivers 300, 340, and 400 of FIGS.3A, 3C, and 4, do not include a decoded de-selection part, these driversmay be part of a larger arrangement of drivers that enable global-leveland/or patch-level decoding of de-selection paths/supplies.

FIGS. 5A-5B are a circuit diagrams showing a memory circuits with fullydecoded and partially-decoded de-selection elements respectivelyincluding multiple de-selection current/voltage supplies, in accordancewith one or more embodiments of the disclosure. As mentioned above inreference to FIG. 1D, in various embodiments sufficient current may beneeded during the stable time of the SET operation. This is particularlytrue because the set current may be supplied by the mid-bias supply tosave power. Thus, in various embodiments, multiple mid-bias currentsources may be provided, as shown in FIGS. 5A and 5B, to enable variousWRITE and READ operation of the memory circuits at lower voltages.

FIG. 5A includes many elements similar to those FIG. 1A described above,thus only the differences are described here. FIG. 5A shows a memorycircuit 500 that includes two each of de-select bit line voltage sources(DESBL1 502 and DESBL2 504) and de-select word line voltage sources(DESWL1 506 and DESWL2 508). In the memory circuit 500, the DESBL1voltage source 502 provides voltage to the drivers comprising n-typeFETs 512 and p-type FETs 510, the DESBL2 voltage source 504 providesvoltage to the drivers comprising n-type FETs 516 and p-type FETs 514,the DESWL1 voltage source 506 provides voltage to the drivers comprisingp-type FETs 520 and n-type FETs 518, and the DESWL2 voltage source 508provides voltage to the drivers comprising p-type FETs 524 and n-typeFETs 522. Multiple current sources/voltage sources thus provideadditional current to enable memory cell operations at lower voltages.

FIG. 5B includes many elements similar to those FIGS. 1A and 5Adescribed above, thus only the differences are described here. FIG. 5Bshows a memory circuit 550 including array-based de-selection paths. TheDESBL1 voltage source 502, the DESBL2 voltage source 504, the DESWL1voltage source 506, and the DESWL2 voltage source 508 are each coupledto multiple bit and word lines through array elements 570 (similar tothose described above in reference to FIG. 4). Further, bit controllines 552 and word control lines 554 may be coupled to, for example,control circuitry. As in memory circuit 500, memory circuit 550 includesmultiple current sources/voltage sources to provide additional currentto enable memory cell operations at lower voltages. The memory circuit550 further illustrates a circuit configuration including array based,non-decoded de-selection paths. In an embodiment, a pattern of arraybased de-selection elements may be desired to enable decoding forodd-even or other such combinations. Multiple voltage/current suppliesmay reduce overall power because only some of the bit lines and/or wordlines will need to transition to the higher levels needed to support therequired currents during WRITE and READ operations.

FIG. 6 is chart showing circuit behavior in the memory cell circuit 102(of FIG. 1B) during a SET function, in accordance with one or moreembodiments of the disclosure. The waveform of FIG. 6 is similar to thevoltage waveform of FIG. 1D. FIG. 6 illustrates four voltage waveforms602, 604, 606, 608 with voltages varying over time between zero voltsand the SELBL voltage, with the mid-bias voltage corresponding to somevoltage between the SELBL voltage and zero volts. Typically, themid-bias is a voltage about half-way between SELBL and zero volts. Alsoshown in FIG. 6 is the state of the WL selection 610 (corresponding toWL select 116), the BL pulse 612 (corresponding to BL select 110, butinverted for clarity), and the de-selection supply bump 614. A memorycontroller can provide control signals for the WL selection 610 and theBL pulse 612.

In FIG. 6, voltages for the bit line 114 and the word line 120 for theselected memory cell are indicated by waveforms 602, 604, respectively.Voltages for the bit line 114 and the word line 120 for one or morede-selected memory cells are indicated by waveforms 608, 606,respectively. As shown, the voltages of both the bit and word lines ofmost of the de-selected cells (waveforms 608, 606) are driven tomid-bias through the entire time period represented. During a timeperiod 624, however, some de-selected bit lines' voltages are increased,as is described below. On the other hand, beginning at a time instantrepresented by a dashed line 616, and during the time period 620, whenWL selection 610 goes high and BL pulse 612 goes low, the waveform 602illustrates that the bit line 114 is pulled to the SELBL voltage source,while the waveform 604 illustrates that the word line 120 is pulled toabout zero volts. At the time of selection, a voltage difference 626exists between a voltage of the word line 120 (of the selected memorycell) and the voltage of the word lines and bit lines of the de-selectedmemory cells. Similarly, voltage difference 627 exists between thevoltage of the selected bit line and the deselected word lines and bitlines. The differences 626 and 627 are chosen to support proper biasingof the cross-point array such that false selection of non-target cellsand proper selection of the target cell can be achieved during arrayoperation. As shown in the time period 620, a snapback event rapidlyoccurs in the selected memory cell, causing a collapsed voltage.

Similar to the waveforms of FIG. 1D, during time period 620 the selectedmemory element exhibits a snapback event, driving the cell into a highconduction, low resistance state. Immediately following the snapbackevent, the bit line 114 is de-selected, as indicated by the BL pulse612, and similar to FIG. 1D. For example, de-selection of the BL pulse612 creates a de-selection voltage potential, or mid-bias voltagepotential (i.e., from mid-bias to zero volts), across the memory cell122. During the time period 622, the selected memory cell enters astable state during which, in some embodiments, the cell may be set.However, in various embodiments, the voltage difference (indicated by628) across the memory cell 122 may not be sufficient to provide thenecessary current to set the memory cell. For example, in an embodimentthe current supply to the memory cells may be limited. To compensate, ata time instant 618, the mid-bias voltage for a portion of the bit linedrivers may be temporarily increased, as indicated by the de-selectionsupply bump 614. During the time period 624, the set current provided tothe selected memory cell is increased as indicated by a voltagedifference 630. As a consequence, as indicated by voltage difference632, for a subset of the memory array, a greater voltage difference iscreated between the target word line and the deselected bit lines andword lines in the array than during time period 622. As this voltagedifference 632 is substantially the same as the difference 626 duringtime period 620, the proper biasing of the cross-point array can bemaintained during the SET operation. Examination of the different biaseselsewhere within the cross-point array will show that none of theconditions for biasing have been violated. Thus, for cases for which thesupply of current is limited, partially decoding the supply to increasevoltage levels for particular memory cells may enable the array to meetwrite requirements.

In an embodiment, the increase in the mid-bias does not necessarilyviolate biasing constraints after selection, because only one word-lineis assumed selected in a cross-point tile and no bit lines are selected.For example, deselecting the target bit line to the mid bias supplyafter the initial trigger of the target cell allows the mid bias to beincreased without incurring a biasing violation elsewhere in the array.In an embodiment, the increase in the mid-bias supply or supplies isenabled only if the snapback event has been detected. Alternatively, themid-bias/de-selection supply increase may occur after a timed event. Themagnitude of the increase may be limited by the biasing constraint ofthe array. The increase in the mid-bias may be limited to only a subsetof bit line drivers.

FIG. 7 is a chart showing circuit behavior in an array-based memory cellduring a SET function, in accordance with one or more embodiments of thedisclosure. For example, the voltage waveform of FIG. 7 may be generatedby the array-based memory circuit 550 of FIG. 5B. FIG. 7 includes fourvoltage waveforms (702, 704, 706, and 708) with voltages varying overtime between zero volts and SELBL, with mid-bias indicating some voltagebetween the SELBL voltage and zero volts. Typically, mid-bias is avoltage about half-way between the SELBL voltage and zero volts. Alsoshown in FIG. 7 is the state of WL selection 710, BL pulse 712(inverted), and de-selection element trigger pulse 714. A memorycontroller can provide the control signals for the WL selection 710, theBL pulse 712, and the de-selection element trigger pulse 714.

In FIG. 7, the bit line and word line levels for the selectedarray-based memory cell are indicated by waveforms 702, 704,respectively. Bit line and word line levels for one or more de-selectedarray-based memory cells are indicated by waveforms 708, 706,respectively. As shown, the voltage levels of both the bit and wordlines of the de-selected cells (waveforms 708, 706) are driven tomid-bias through the entire time period represented. On the other hand,beginning at time instant 716 (represented by a dashed line), and duringthe time period 720, when WL selection 710 goes high and BL pulse 712goes low, the waveform 702 indicates that the bit line is pulled to theSELBL voltage, while the waveform 704 illustrates that the word line ispulled to zero volts. During the time period 720 the selected memoryelement exhibits a snapback event, driving the cell into a highconduction, low resistance state.

Then, during time period 722, the selected memory cell enters a stablestate during which, in some embodiments, the cell may be set. However,in various embodiments, the voltage difference (indicated by 726) acrossthe memory element may not be sufficient to provide the necessarycurrent to set the memory cell. To compensate, during time period 715the snapback event is detected, and as a result at time period 718(indicated by a dashed line) a de-selection element trigger pulse 714 isapplied to the selected cell. The de-selection element trigger pulse 714comprises a temporary bump in the mid-bias supply voltage to thede-selection array element associated with the bit line of the selectedcell (for example, the particular array element 570, as shown in FIG.5B) or to a group of such bit-lines associated with the selected cell.The snapback event may be detected by, for example, lower resistanceand/or high current flow in the selected cell. In an embodiment, thetrigger pulse 714 is only applied after the snapback event has beendetected. Because the bit line voltage (waveform 702) is already low,the pulse applied to the mid-bias supply of the particular bit linede-selection element may trigger a snapback event for that particularde-selection element, forcing the element into a high conduction state.This enables additional current capacity to the target selected cellduring the time period 724, as indicated by the increased voltagedifference 728. The memory cell may therefore have sufficient current toaccomplish the SET operation (or other operation) during the time period724.

In an embodiment, the mid-bias supply may be pulsed on one or more bitline de-selection elements depending on the decoding configuration ofthe memory circuit.

Thus, as shown in FIG. 7, for the embodiment including array-basedde-selector elements with threshold characteristics, a partially decodedsupply may be pulsed to trigger the de-selector element into a lowresistance state, which may then improve a loadline supply (currentsupply) to the selected memory cell.

In an embodiment, the WRITE operation techniques described in thepresent disclosure may be particularly applicable to READ operationswhen the READ operations use fixed current and/or require a stablecurrent for a particular period of time.

In various embodiments and various memory technologies, when a memorycell is selected there is a particular minimum current required tomaintain the selected cell in a conducting, low resistance, state (an“on state”). While the techniques described herein maintain the selectedmemory cells in the on state, various embodiments may additionallyemploy other techniques. For example, in an embodiment a current controlcircuit may be applied in order to maintain enough current to maintainan “on state” in a selected cell. Various memory technologies, includingphase change materials, have a “recovery period” or “threshold recoveryeffect.” In these memory technologies, the threshold of the memory cellimmediately after trigger is reduced and may require a finite period oftime before recovering to its initial value. Thus, in these technologiesthe threshold of the memory cell soon after a snapback event may be verylow. For example, when a particular cell requires eight volts to snapback, if that cell drops out of conduction but a voltage is reappliedvery quickly, the threshold may be one or two volts. Thus, in someembodiments, when a reduced voltage is applied to a selected memory cellwithin a short period of time, the selection of the cell may bere-obtained.

One embodiment includes a processor configured to host one or moreapplications and to initiate write and/or read commands to a memorydevice controller to provide access to a memory array.

One embodiment includes an apparatus comprising: a memory cellelectrically coupled to a first line and to a second line; and a drivercircuit electrically coupled to the first line and to the second line,the driver circuit configured: to provide both a selection mode ofoperation and a de-selection mode of operation for the memory cell; toreceive at least a first voltage reference, a second voltage reference,and a third voltage reference, wherein each of the first voltagereference, the second voltage reference, and the third voltage referenceis different from one another; to bias the memory cell based on thefirst voltage reference and the third voltage reference when in theselection mode of operation; and to bias the memory cell based on atleast the second voltage reference when in the de-selection mode ofoperation.

One embodiment includes an electronically implemented method ofselecting a memory cell, the method comprising: receiving at least afirst voltage reference, a second voltage reference, and a third voltagereference, wherein each of the first voltage reference, the secondvoltage reference, and the third voltage reference is different from oneanother; biasing a memory cell with a selection voltage based on thefirst voltage reference and the third voltage reference to select thememory cell; and biasing the memory cell with a de selection voltagebased on at least the second voltage reference to de select the memorycell.

One embodiment includes a system comprising: a processor configured toissue at least one of a read operation or a write operation; a memorydevice comprising: a memory array comprising a memory cell electricallycoupled to a first line and to a second line; and a driver circuitelectrically coupled to the first line and to the second line, thedriver circuit configured: to provide both a selection mode of operationand a de selection mode of operation for the memory cell; to receive atleast a first voltage reference, a second voltage reference, and a thirdvoltage reference, wherein each of the first voltage reference, thesecond voltage reference, and the third voltage reference is differentfrom one another; to bias the memory cell based on the first voltagereference and the third voltage reference when in the selection mode ofoperation; and to bias the memory cell based on at least the secondvoltage reference when in the de selection mode of operation; and a busconfigured to carry information associated with the at least one of theread operation or the write operation.

Semiconductor memory devices are ubiquitous in apparatuses such asdigital electronics devices and various systems (as used herein, an“apparatus” can interchangeably refer to, for example, circuitry, amemory device or combination of memory devices, an electronic device orcombination of electronic devices, or a system or combination ofsystems). These various systems can include a process configured toexecute instructions and/or access data stored on the memory devices.Examples of apparatuses include, but are not limited to, desktopcomputers, workstations, servers, tablets, laptop computers, digitalcameras, video cameras, digital media players, personal digitalassistants, smart phones, mobile phones, navigation devices,non-volatile storage products, kiosks, automotive products, and thelike.

The foregoing description and claims may refer to elements or featuresas being “connected” or “coupled” together. As used herein, unlessexpressly stated to the contrary, “connected” means that oneelement/feature is directly or indirectly connected to anotherelement/feature, and not necessarily mechanically Likewise, unlessexpressly stated to the contrary, “coupled” means that oneelement/feature is directly or indirectly coupled to anotherelement/feature, and not necessarily mechanically. Thus, although thedrawings illustrate various examples of arrangements of elements andcomponents, additional intervening elements, devices, features, orcomponents may be present in an actual embodiment.

As used herein, a “node” refers to any internal or external referencepoint, connection point, junction, signal line, conductive element, orthe like at which a given signal, logic level, voltage, data pattern,current, or quantity is present.

Various embodiments have been described above. Although described withreference to these specific embodiments, the descriptions are intendedto be illustrative and are not intended to be limiting. Variousmodifications and applications may occur to those skilled in the art.

What is claimed is:
 1. A method, comprising: initializing a memory cellcoupled with a first select line and a second select line, wherein theinitializing comprises coupling the first select line with a firstvoltage source; detecting a transition of the memory cell from a firststate to a second state that has a lower resistance than the firststate; and selecting the memory cell based at least in part on detectingthe transition, wherein the selecting comprises coupling the firstselect line with a second voltage source that is different from thefirst voltage source.
 2. The method of claim 1, wherein the secondvoltage source has a lower voltage magnitude than the first voltagesource.
 3. The method of claim 1, wherein the second select line iscoupled with a third voltage source during the initializing and duringthe selecting.
 4. The method of claim 3, wherein the second voltagesource has a voltage that is between a voltage of the first voltagesource and a voltage of the third voltage source.
 5. The method of claim3 further comprising: deselecting the memory cell by coupling the secondselect line with a fourth voltage source that is different from thethird voltage source.
 6. The method of claim 5, wherein the fourthvoltage source has a voltage that is between a voltage of the firstvoltage source and a voltage of the third voltage source.
 7. The methodof claim 5, wherein the second voltage source and the fourth voltagesource are the same voltage source.
 8. The method of claim 1, whereinthe selecting is performed within a recovery period of the memory cellafter detecting the transition.
 9. The method of claim 1, furthercomprising: performing an access operation on the memory cell based atleast in part on a current associated with the initializing.
 10. Themethod of claim 1, further comprising: determining whether theinitializing performs an access operation on the memory cell; andperforming, based at least in part on the determination, the accessoperation on the memory cell by increasing a voltage magnitude of thesecond voltage source.
 11. An apparatus, comprising: a memory arraycomprising a plurality of memory cells, each of the plurality of memorycells having a first portion electrically coupled with a line of a firstplurality of lines and having a second portion electrically coupled witha line of a second plurality of lines; and a driver circuit configuredto: initialize a first memory cell of the plurality of memory cells,wherein the initializing comprises coupling a respective line of thefirst plurality of lines for the first memory cell with a first voltagesource; detect a transition of the first memory cell from a first stateto a second state that has a lower resistance than the first state; andselect the first memory cell based at least in part on detecting thetransition, wherein the selecting comprises coupling the respective lineof the first plurality of lines for the first memory cell with a secondvoltage source that is different from the first voltage source.
 12. Theapparatus of claim 11, wherein the second voltage source has a lowervoltage magnitude than the first voltage source.
 13. The apparatus ofclaim 11, wherein the driver circuit is configured to couple arespective line of the second plurality of lines for the first memorycell with a third voltage source during the initializing and during theselecting.
 14. The apparatus of claim 13, wherein the second voltagesource has a voltage that is between a voltage of the first voltagesource and a voltage of the third voltage source.
 15. The apparatus ofclaim 14, wherein the driver circuit is configured to: deselect thefirst memory cell, wherein the deselecting comprises coupling therespective line of the second plurality of lines with a fourth voltagesource that is different from the third voltage source.
 16. Theapparatus of claim 15, wherein the second voltage source and the fourthvoltage source are the same voltage source.
 17. The apparatus of claim11, wherein the driver circuit is configured to: deselect a secondmemory cell of the plurality of memory cells during the initializing andduring the selecting, wherein deselecting the second memory cellcomprises coupling a respective line of the first plurality of lines forthe second memory cell with the second voltage source.
 18. An apparatus,comprising: a memory cell, coupled with a first select line and a secondselect line; and a driver circuit configured to: initialize the memorycell, wherein the initializing comprises coupling the first select linewith a first voltage source; detect a transition of the memory cell froma first state to a second state that has a lower resistance than thefirst state; and select the memory cell based at least in part ondetecting the transition, wherein the selecting comprises coupling thefirst select line with a second voltage source that is different fromthe first voltage source.
 19. The apparatus of claim 18, wherein thesecond voltage source has a lower voltage magnitude than the firstvoltage source.
 20. The apparatus of claim 18, wherein the drivercircuit is configured to couple the second select line with a thirdvoltage source during the initializing and during the selecting.